1. Field of the Invention
The present invention relates to a self-poled electro-optic (EO) and nonlinear optical (NLO) polymer, a method of manufacturing the polymer, and a film manufactured from the polymer. More specifically, the present invention relates to a self-poled EO and NLO polymer, a method of manufacturing the polymer, and a film manufactured from the polymer. Organic chromophores chemically attached to the polymer are one-directionally oriented in 3-dimensional space due to the configuration control of the polymer backbone chemical structure. A poling process is not required when the film is formed.
2. Discussion of Related Art
Recently, in line with the ongoing development of devices for high-speed and high-bandwidth data transmission, EO and NLO materials that can be used in the field are in high demand, and researches on their utilization have actively been in progress. Materials currently used for high-speed optical communication devices include inorganic crystals such as LiNbO3 and InGaAsP. The cost for manufacturing such materials is very high even if they show stable optical nonlinearity. The manufacturing process is also difficult and time-consuming.
On the other hand, organic materials, polymers in particular, with EO and NLO properties have been developed for about 20 years. Compared with the aforementioned inorganic materials, they are easy to be synthesized and processed. In addition, their optical properties such as refractive index, optical coefficient, and absorption wavelength can be controlled as needed. An organic polymer material having EO and NLO properties is manufactured by chemically introduction of organic chromophores into a polymer backbone. Organic chromophores have conjugation on which electrons can move. It shows a dipole moment by introducing an electron-donating and electron-releasing group to the molecule, and is poled with applied electric field. Introducing the organic chromophores into polymer gives the polymer system electro-optic and nonlinear optical properties.
EO and NLO polymers can be largely classified into four groups according to the relationship between the polymer and the organic chromophore (G. A. Lindsay, “Second-Order Nonlinear Optical Polymers: An Overview”, ACS Symp. Ser. 60, G. A. Lindsay and K. D. Singer eds., ACS, 1995, Chapter 1).
The first group is host-guest type polymers. These polymers are prepared by dispersing an organic chromophore into a polymer matrix. Its preparation is simple. Once the organic chromophore is well diffused into the polymer matrix, the poling effect can be maximized due to free motion of the organic chromophore molecules in the polymer. However, the optical nonlinearity significantly decrease due to the free motion of molecules at high temperatures during an optical device is manufactured. In addition, the glass transition temperature (Tg) of the polymer decreases with increasing the content of the organic chromophores. Optical loss originates from the organic chromophore domains, resulting in optical loss.
The second group is side chain-type polymers. This type of polymer is developed to overcome the problem of the host-guest polymer by chemically attaching the organic chromophore to a polymer main chain. Further, separation between the organic chromophore and polymer matrix is prevented and a proper Tg of the polymer is obtained to achieve high temperature stability of the optical nonlinearity. The Tg of the EO polymer obtained using this method is preferably in the range of 150° C. to 200° C. Poling efficiency is maximized around Tg where molecular motion in the EO polymer is high. The optical nonlinearity obtained at below 150° C. may decay during a device is processed (The processing temperature is between about 80° C. and 100° C.). The organic chromophore can be decomposed during poling above 200° C. (M. H. Lee et al., “Polymeric Electrooptic 2×2 Switch Consisting of Bifuraction Optical Active Waveguides and a Mach-Zehnder Interferometer”, IEEE J. on Selected Topics in Quantum Electronics, 7, 812, 2001).
The third group is main chain-type polymers, obtained by incorporation of a nonlinear optical organic chromophore into a polymer main chain. As anticipated, molecular mobility of this type polymer system decreases compared with the side-chain type polymer system so that the EO effect is highly thermostable.
The fourth group is crosslinked type polymers. This type polymer system is used to improve the thermal stability of EO and NLO effect after poling the host-guest and side-chain type polymers with low Tg. In this method, the polymer main and side chains are crosslinked during or after poling the EO and NLO polymer. After the EO polymer is crosslinked, the molecular motion of the organic chromophore is reduced so that the EO effect can be significantly maintained even at high temperature. In general, the polymer main chain is thermo- or photo-crosslinked in the presence of a catalyst. However, after the crosslinking reaction, the unreacted cross-linkers or catalyst remain, which limits use of the crosslinked polymer system for optical devices (U.S. Pat. No. 5,420,172, U.S. Pat. No. 5,776,374).
Among the above four types polymer system, it is well known that the side chain type polymer system is the most suitable for optical devices in terms of poling effect and thermal stability of the optical nonlinearity.
Korean Patent Application No. 2003-28187 entitled “Side Chain Type Polyamide Ester as Electro-Optic and Nonlinear Optical Polymer, Manufacturing Method Thereof, and Film Manufactured Therefrom” filed May 2, 2003, by the present inventors, discloses EO or NLO side chain polymer manufactured by reaction of an organic chromophore with polyamic acid, which is a precursor of polyimide.
Further, all EO and NLO polymers obtained from any of the above four methods are necessarily applied with an external electric field while the polymers is heated to around their Tg, to obtain high EO and NLO polymer property. The organic chromophores in the film are noncentrosymmetric. In fact, in manufacturing a polymer optical device such as an optical modulator or an optical switch, an EO and NLO polymer film is between a lower cladding layer and an upper cladding layer, and an electrode is deposited on the upper cladding layer to apply an electric field in a direction of a film thickness for poling the polymer waveguide. An external electric field is applied according to the thickness of the cladding and core layers. Therefore, the poling effect is especially affected by electric conductivity between the cladding and the polymer core. In order to maximize the poling effect, it is important to develop the cladding layer having lower conductivity than that of the electro-optic and nonlinear optical polymer. That is another research subject.
If the organic chromophores in the polymer film spontaneously is aligned themselves noncentrosymmetrically without a poling process when EO and NLO polymer is spin-coated, all processes related to the poling process during manufacture of the optical device can be omitted so that the overall process is simplified and requirements for cladding materials are simplified. In general, such a natural noncentrosymmetric EO and NLO polymer film is obtained using a langmuir-blodgett (LB) process or a layer-by-layer self-assembly process (Webin Lin et al, Supramolecular approaches to Second-Order Nonlinear Optical Materials, Self-Assembly and Microstructural Characterization of Intrinsically Acentric [(aminophenyl)azo]pyridinum Superlattices, J. Am. Chem. Soc., 118, 8034, 1996), in which single molecular layers are stacked layer-by-layer due to molecular force of the organic chromophores. However, it is not easy to obtain a polymer film with a thickness of 3 to 4 μm or more required in real optical devices.
The present inventors found that the EO and NLO side chain polyamide ester disclosed in Korean Patent Application No. 2003-28187 has three diacid-diester isomers in the polymer repeating unit, so that when the isomer with one-directionally oriented chromophore in 3-dimensional space is separated and used for polymerization, high EO and NLO properties can be naturally achieved without a poling process. This invention renders a poling process unnecessary and the EO film with a desired thickness is easily formed by a simple spin-coating method.